Weight scale for atoms could map 'island of stability'

Hunting for the universe's heaviest atoms just got a little easier, thanks to a new technique that directly measures the mass of elements heavier than uranium. The method could help find an "island" of unusually stable elements that is thought to extend beyond the current end of the periodic table.

Uranium, which contains 92 protons, is the heaviest element known to occur in nature. But researchers have synthesised a number of even heftier elements, with as many as 118 protons.

These extreme atoms are quite short-lived – many fall apart just milliseconds after they are created. But nuclear theorists suspect that a class of 'super-heavy' atoms, boasting the right combination of protons and neutrons, could have lifetimes of decades or longer (see Hunting the biggest atoms in the universe).

Elements in this so-called island of stability could act as powerful nuclear fuel for future fission-propelled space missions. They might also be exhibit useful new chemical properties. Element 114, for example, has shown hints that it behaves like a gas at room temperature even though it should be a member of the lead family on the periodic table.

But no one knows where the island of stability lies; some models predict it is centred on atoms with 114 protons, while others put it near atoms with 120 or 126 protons.

The uncertainty arises because it is not clear how strongly the nuclei of super-heavy atoms are bound together, and thus how stable they are. Determining this 'binding energy' has been difficult because super-heavy atoms are short-lived; experimentalists have had to estimate it by looking at the atoms produced when a super-heavy atom breaks apart.

Now a team led by Michael Block of the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany has demonstrated a direct way of measuring the mass of particles heavier than uranium.

Binding energy

Because mass and energy are equivalent, as described by Einstein's famous equation E = mc2, determining the mass of an atom indicates how strongly its nucleus is bound together.

To make the mass measurement, the team used a device called a Penning trap, which employs electric and magnetic fields to confine atoms.

The trap was used to weigh atoms of nobelium, an element that contains 102 protons, 10 more than uranium. Like other 'artificial' elements, the nobelium atoms were created by colliding a stream of lighter atoms with a target.

The key advance was finding a way to slow down the nobelium atoms before they entered the trap – a feat the team accomplished by first injecting the atoms into a chamber filled with helium gas.

Hard to produce

Measuring the mass of other elements could help theorists begin to differentiate between competing models for the structure of super-heavy nuclei. "It will [give us] important experimental evidence that can be used to confront nuclear structure theories," says Walter Loveland of Oregon State University in Corvallis, who was not associated with the study.

"There's nothing in the method that would prevent you from moving significantly up the periodic table," he says. But in practice, it will likely be difficult to do with the heaviest atoms because they are so hard to synthesise in the first place. For example, nobelium atoms can be created at the rate of one per second by shooting other atoms at a target, but super-heavy elements, atoms with 104 or more protons, are currently produced at the rate of one per week, if that.

If the technique can be extended to super-heavy elements, it could eventually help identify inhabitants of the island of stability when they are created.

These atoms would be produced in such small quantities and would decay so slowly that they would be extraordinarily hard to detect by their decay products alone. "We need another way to identify them," Block told New Scientist. "The mass in principle can be considered as unique as a fingerprint."

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The mass of an atom heavier than uranium has been measured for the first time (Image: bjearwicke/stock.xchng)